Since the 1960s, scientists and seafarers working in the Southern Ocean have puzzled over the rhythmic quacking sound heard everywhere from the southernmost tip of the Atlantic to the coast of western Australia. New acoustic recordings have finally solved that decades-long mystery: The strange, aptly nicknamed “bio-duck” sounds come from the Antarctic minke whale (Balaenoptera bonaerensis).

About a year ago, a team led by Denise Risch of NOAA’s Northeast Fisheries Science Center deployed acoustic tags onto two minke whales in Wilhelmina Bay along the western Antarctic Peninsula. To make them stick, the scientists outfitted the tags with suction cups, went out into the bay in an inflatable boat, and used a hand-held carbon fiber pole to attach the tags to the cetaceans.

Over the course of a couple dozen hours of recordings, the team captured 32 clear minke whale calls, including a low-frequency sound with a repetitive pattern: a series of pulses with a 3.1-second interval between the start of each series. When they compared these calls to older recordings, the team concluded once and for all that minke whales are the source of the bio-duck sounds.

In addition to answering a half-century-old question, the recordings clarify the distribution and behavior of this poorly understood baleen whale species living in a rapidly changing sea-ice environment. It turns out that some minke whales stay in ice-covered Antarctic waters year-round, while others opt to migrate to lower latitudes. That explains why the bio-duck can be heard in higher and lower latitudes simultaneously during wintertime in the Southern Hemisphere, though the researchers still don’t know why or even how the calls are made.

Minke whales are also the primary target of Japan’s controversial Antarctic whale hunt. Just two weeks ago, Japan’s Institute of Cetacean Research said it plans to resume research whaling in the region next year. However, to comply with this month’s ruling from the International Court of Justice, Japan will need to provide valid scientific reasons for its hunting practices. — Janet Fang | 24 April 2014